The Data.Logic.Propositional module provides a set of functions for
parsing, manipulating and generating truth tables for expressions in
classical propositional logic.

The core of the API is the Expr data type, which has constructors for all
the usual expression forms: variables, standing for atomic propositions;
negation, the only unary connective; and the binary connectives of
conjunction, disjunction, material implication and logical equivalence.

In order to interpret an expression, a mapping from variables to truth
values needs to be provided. Truth values are compositional; that's to say,
the value of a composite expression (any expression which is not atomic)
depends on the truth values of its component parts. For example, the Haskell
expression below would evaluate to False.

The parseExpr function accepts the name of a source, and a string to be
parsed, and attempts to parse the string as a logical expression of the
following forms, where φ and ψ are metalinguistic variables
standing for any valid expression.